These experiments and many others like them conducted on soldiers and Spanish immigrants proved that this particular mosquito would transmit the disease under certain conditions.
1. The mosquito must bite the patient during the first three days of the fever; after that a yellow fever patient cannot infect a mosquito.
2. A period of twelve days must elapse before the mosquito is able to infect another person. After that she may infect anyone she may bite; that is, the germs remain virulent during the rest of the mosquito's life. The French Yellow Fever Commission working in Rio de Janeiro claim that the first generation of offspring from such an infected mosquito is capable of causing the disease after they are fourteen days in the adult condition.
The next step was to ascertain whether the disease could be contracted in any other way than by the bites of infected mosquitoes. A camp named Camp Lazear was established and the following tests made: A mosquito-proof building of one room was completely divided by a wire screen from floor to ceiling. In one room fifteen mosquitoes that had previously bitten yellow fever patients and had undergone the proper period of incubation were liberated. In this room a non-immune exposed himself so that he was bitten by several of the insects. A little later the same day and again the next day the mosquitoes were allowed to feed on him for a few minutes. Five days later, the usual incubation period, he developed yellow fever.
At the same time that he entered the building two other non-immunes entered the other compartment where they slept for eighteen nights separated from the mosquitoes by the wire screen. They showed no signs of taking the fever.
In another mosquito-proof house two soldiers and a surgeon, all non-immunes, lived for twenty-one days. From time to time they were supplied with soiled articles of bedding, clothing, etc., direct from the yellow fever hospital in the city. These articles had been soiled by the urine, fecal matter and black vomit obtained from fatal and other cases of yellow fever. These articles were handled and shaken daily, but no disease developed among the men and at the end of the twenty-one days, two other non-immunes relieved them and handled a new supply of clothing in the same way, sleeping between the same sheets that had been used by a patient dying of yellow fever and exposing themselves in every possible way to the soiled clothing. But no disease developed. That these men were susceptible was shown later by inoculating some of them, when they developed the disease.
In another experiment certain men in a camp allowed themselves to be bitten by mosquitoes that had passed through the proper period of incubation and every one of them and no others contracted the disease. It was also shown that a mosquito was capable of communicating the disease as long as fifty-seven days after it had bitten a yellow fever patient. Another set of experiments showed that a subcutaneous injection into a non-immune of a very small quantity of blood from the veins of a yellow fever patient in the first two or three days of the disease would produce the fever.
SUMMARY OF RESULTS
Since that time much other work has been done by independent workers as well as by French and English Commissions both working at Rio de Janeiro. The results of their investigation are practically the same and may be summed up as follows: